Antenna diversity method with fallback and weighting

ABSTRACT

In a radio communication station having N antennas an antenna diversity system and a gain controllable amplifier the gain of the gain controllable amplifier is set by using a first antenna, the signal levels of signals received at the antennas is measured, a group of antennas is determined having a signal level which exceeds the signal level of the first antenna by no more than a predetermined value (X), and out of said group of antennas the antenna with the highest signal level is selected out or the first antenna is selected out if said group is empty.

PRIORITY

This application claims foreign priority to Swedish application number SE0400492-5 filed Feb. 27, 2004.

TECHNICAL FIELD

The present invention relates in general to a method of receiving and processing signals in mobile communication systems and in particular to a method of selecting an antenna in a receiver section of a radio communication station wherein the radio communication station comprises a number N of antennas and the receiver section comprises an antenna diversity system and a gain controllable amplifier.

BACKGROUND

The key idea when using switched antenna diversity is to have more than one antenna in the receiver. For each received frame the signal level of the received signal, in particular the received signal strength (RSS) is measured for all or a selection of the antennas in the receive path. These RSS measurements are made during the initial preamble part of the frame that is used for receiver training. Before the data part of the frame is entering the receiver, the antenna with the strongest RSS is selected for reception of the rest of the frame. In this way an increased receiver performance will be gained compared to using only one antenna.

During the preamble different tasks have to be performed in the receiver. The most important task is to adjust the signal level to be within the dynamic range of the receiver by setting the gain of the amplifier. This is done by measuring the RSS of the amplified signal and adjusting the gain of the amplifier repeatedly until the signal level is in accordance with the dynamic range of the receiver. This procedure is done using one (i) of the N antennas of the station.

After the gain of the amplifier has been set, the antenna diversity measurements are performed. The last RSS measurement done during the gain setting can be used as RSS estimate for antenna i. For the remaining antennas separate RSS measurements must be performed.

For simplicity it can be assumed that the station comprises two antennas where antenna 1 is used for the gain calculations. There are two situations which can arise after the RSS measurements have been made for both antennas:

-   -   1. RSS for antenna 1 is higher than for antenna 2 (RSS₁>RSS₂)     -   2. RSS for antenna 1 is higher than for antenna 1 (RSS₂>RSS₁)

In case 1 above the gain setting is already as good as it can be since the best antenna is the same antenna as that has been used for gain calculations. In case 2 the antenna not used for gain calculations is found to be the best one and if the difference between the two antennas is large we will need to recalculate the gain again otherwise we might experience a reduced performance compared to always use antenna 1.

The problem is that gain recalculations take time and time is a very limited resource during the preamble.

A further problem of antenna diversity lies in the following. When using more than one antenna in the receiver it is possible that the noise level on all antennas is not equal. The cause of this can be a bad placement of the antennas or that one antenna experiences stronger adjacent channel disturbance. With weak signal levels the received noise level will be so high that it heavily affects the antenna selection. We will choose the antenna with the highest noise level instead of the one with the strongest signal level which would be the correct choice.

The best solution would be if it was possible to estimate the noise level for each received frame and take this into account when selecting antenna. This is however both difficult and expensive in terms of hardware costs.

SUMMARY OF EMBODIMENTS

It is therefore one object of the present invention to provide a method of selecting an antenna in a receiver section of a radio communication station wherein said station comprises a number of antennas and said receiver section comprises an antenna diversity system and a gain controllable amplifier, wherein said method allows to select an antenna without conducting gain recalculations. It is a further object of the present invention to take into account static noise levels experienced by each antenna.

The present invention, according to an embodiment, relates to a method of selecting an antenna in a receiver section of a radio communication station, said station comprising a number (N) of antennas and said receiver section comprising an antenna diversity system and a gain controllable amplifier, said method comprising the steps of:

-   -   setting the gain of the gain controllable amplifier by using a         first antenna of the N antennas;     -   measuring the signal levels of signals received at the antennas,     -   determining a group of antennas having a signal level which         exceeds the signal level of the first antenna by no more than a         predetermined value (X),     -   selecting out of said group of antennas the antenna with the         highest signal level or selecting the first antenna if said         group is empty.

The present invention, according to an embodiment, offers a solution for the above indicated situation wherein a first antenna was used for the gain adjustments and for anyone of the other antennas a signal level or RSS is measured which is higher than that of the first antenna. It is an essential idea of the present invention to determine a group of antennas whose signal levels exceed the signal level of the first antenna by no more than a predetermined value (X) and to select out of this group the antenna with the highest signal level. If the group is empty, the first antenna is selected out.

In the second step of measuring the signal levels it is not necessary to measure the signal level of the first antenna as the signal level value measured in the preceding setting step can be used.

The most advantageous aspect of the invention according to an embodiment is that no recalculation of the gain of the gain controllable amplifier is carried out after the RSS measurements. Even in the case that one or more antennas have higher signal level than the first antenna which was used for the gain adjustment no readjustment of the gain is conducted and the performance of the antenna diversity system is non the less kept at high level.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in the following in more detail in conjunction with the accompanying drawings which show

FIG. 1 an outline of a receiver with two antennas;

FIG. 2 an outline of a typical frame structure of a WLAN standard IEEE 802.11a/g;

FIG. 3 a simplified outline of the method steps of an embodiment of the present invention; and

FIG. 4 a receiver structure for utilizing the inventive method.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 3 shows again a typical frame structure as used in the WLAN standard IEEE 802.11a/g. The preamble contains essentially training symbols to be evaluated in the receiver for purposes of equalization. After the preamble the information data will be transmitted. It is to be noted that the length of the preamble and of the data block are not to scale on the time axis.

The inventive method is to be carried out during the preamble. In the gain setting step the signal level of one antenna, i.e. its RSS value is measured after the amplifier as shown in FIG. 4 and thereafter the gain of the gain controllable amplifier is set. This is repeated in an iterative manner until a gain value of the gain controllable amplifier which adequately corresponds to the dynamic range of the receiver is calculated and set. Thereafter, the antenna is switched and RSS values of the other antennas are measured wherein it is not necessary to measure again the RSS value of the first antenna which was used for the gain setting steps. In the last method step an antenna is selected according to the selecting step as outlined above. This selected antenna is then used for receiving the information data.

In a hardware description language like pseudo code the implementation would have the following form:

antenna_selection = 1; RSS_selection = RSS1; for (i = 2; i ≦ N; i++)   if ((RSSi − RSS1) > X) then     //too large signal, do not use this antenna,     keep old one     antenna_selection = antenna_selection     RSS_selection = RSS_selecton; else   if RSSi > RSS_selection then     //new antenna better, select it     antenna_selection = i;     RSS_selection = RSSi;   end if end if.

Those lines with a preceding “//” contain descriptive text.

In many cases the radio communication station comprises two antennas and the gain adjustments are carried out with a first antenna which experiences an RSS value of RSS1. The present invention according to an embodiment provides for a method of selecting an antenna without the need for a recalculation of the gain in the case that RSS2 is larger than RSS1. The present invention was based on the assumption that there are some margins in the receiver so that as long as RSS2 not exceeds RSS1 with more than X dB no gain adjustment is needed. If RSS2 is more than X dB stronger RSS1 we fall back to use the first antenna, i.e. antenna 1, since this is the best antenna when there is no additional time for gain adjustments available. An appropriate value of X is dependent on the radio standard and implementation but is probably in the range of 3 to 10 dB. X can be stored in a register making it possible to adapt it for different environments.

In the case of two antennas (N=2) in a hardware description language pseudo-code, the implementation would be:

if (RSS2 − RSS1) > X then   antenna_selection = 1; else   if RSS2 > RSS1 then     antenna_selection = 2;   else     antenna_selection = 1;   end if; end if.

Antenna 2 is selected out if RSS2 exceeds RSS1 by no more than X. In the general terms of claim 1 the group of antennas as determined in the third step only consists of antenna 2.

A further advantageous embodiment of the present invention addresses to the second problem outlined above which is related to the fact that the noise level is not equal on all antennas used. In the preferred embodiment the static noise level experienced by each antenna is more or less continuously estimated. It will not be discussed how these static noise level estimations are carried out as this is common knowledge to a skilled person. In general it is possible to estimate the noise level during the reception of a frame and then to average over several frames to find out the static noise level. The frame based noise level estimation is for instance already implemented in WLAN products available on the market.

According to this preferred embodiment the static noise level estimations are taken into account when performing the antenna diversity method. Accordingly for each antenna this static noise level is determined and subtracted from the measured RSS values. In particular in the above pseudo code descriptions the terms RSS1, RSS2, RSSi and RSS_selection are replaced by the terms RSS1 N1, RSS2 N2, RSSi Ni and RSS_selection-Nselection, where N1, N2, Ni and Nselection are estimated static noise levels for respective antennas. In practice each antenna therefore is provided with a “penalty” or is “weighted” dependent on experienced noise level.

In the case of two antennas, the pseudo code description can be as follows:

if (RSS2 − RSS1) > X then   antenna_selection = 1; else   if RSS2 > RSS1 − (N1 − N2) then     antenna_selection = 2;   else     antenna_selection = 1;   end if; end if.

The difference (N1−N2) between the noise levels N1 and N2 experienced by the two antennas is preferably stored in a register. 

1. A method of selecting an antenna in a receiver section of a radio communication station, said station comprising at least first and second antennas and said receiver section comprising an antenna diversity system and a gain controllable amplifier, said method comprising: measuring an actual signal level of a signal received at the first antenna; setting the gain of the gain controllable amplifier based on the signal level of the signal received at the first antenna; and after the gain of the gain controllable amplifier is set, performing the following steps: measuring an actual signal level of a signal received at the second antenna; selecting the first antenna if the signal level of the second antenna exceeds the signal level of the first antenna by more than a predetermined value (X); selecting the first antenna if the signal level of the first antenna exceeds the signal level of the second antenna; selecting the second antenna if the signal level of the second antenna exceeds the signal level of the first antenna by no more than the predetermined value (X); and maintaining the gain of the gain controller amplifier as set based on the signal level of the first antenna regardless of which antenna is selected.
 2. A method according to claim 1, wherein the signal levels are measured by an RSS method.
 3. A method according to claim 2, wherein the method is implemented in accordance with WLAN standard IEEE 802.11a/g.
 4. A method according to claim 3, wherein the method is carried out during a preamble of a frame as defined in the WLAN standard.
 5. A method according to claim 1, wherein X is a value in the range of 3 to 10 dB.
 6. A method according to claim 1, wherein the method is implemented in accordance with WLAN standard IEEE 802.11a/g.
 7. A method according to claim 6, wherein the method is carried out during a preamble of a frame as defined in the WLAN standard.
 8. A method according to claim 1, further comprising: measuring static noise levels for each antenna; and weighting the signal levels with the static noise levels.
 9. A method according to claim 8, wherein the method is implemented in accordance with WLAN standard IEEE 802.11a/g.
 10. A method according to claim 9, wherein the method is carried out during a preamble of a frame as defined in the WLAN standard.
 11. A method according to claim 8, wherein weighting the signal levels with the static noise levels comprises subtracting the static noise level from the signal level for each antenna.
 12. A method according to claim 8, wherein the second antenna is selected if the signal level of the second antenna exceeds the signal level of the first antenna minus a difference between the static noise levels for the first and second antennas, otherwise the first antenna is selected.
 13. A method according to claim 8, further comprising storing the static noise levels in a register.
 14. A method according to claim 13, wherein the static noise levels are stored in the register as a difference between the static noise levels for different ones of the antennas.
 15. A device for selecting an antenna in a receiver section of a radio communication station, the receiver station having at least first and second antennas, the device comprising: a gain controllable amplifier having a settable gain; and an antenna diversity system operable to measure an actual signal level of a signal received at the first antenna, set the gain of the gain controllable amplifier based on the signal level of the signal received at the first antenna, and after the gain of the gain controllable amplifier is set: measure an actual signal level of a signal received at the second antenna; select the first antenna if the signal level of the second antenna exceeds the signal level of the first antenna by more than a predetermined value (X); select the first antenna if the signal level of the first antenna exceeds the signal level of the second antenna; select the second antenna if the signal level of the second antenna exceeds the signal level of the first antenna by no more than the predetermined value (X); and maintain the gain of the gain controller amplifier as set based on the signal level of the first antenna regardless of which antenna is selected. 